Rebuilding a 1975 Case 850 Dozer and Diagnosing Torque Converter Failure

[MikePhua]

The Case 850 Series and Its Historical Role
The Case 850 dozer was introduced in the early 1970s by J.I. Case Company, a pioneer in construction and agricultural machinery since 1842. The 850 series was designed as a mid-size crawler dozer, offering a balance between maneuverability and pushing power. It featured a torque converter transmission, hydraulic blade control, and a rugged undercarriage suitable for grading, land clearing, and light excavation. The 1975 model, part of the early production run, was known for its mechanical simplicity and field-serviceable components.
During its production years, the Case 850 sold widely across North America, particularly to small contractors and municipal fleets. Its popularity stemmed from its affordability and reliability, though many units today require extensive restoration due to age and wear.
Terminology annotation:

• Torque converter: A fluid coupling between the engine and transmission that multiplies torque and allows smooth gear engagement.
  
• Flex plate: A thin metal disc connecting the engine crankshaft to the torque converter, transmitting rotational force.
  
• Charge pump: A hydraulic pump that supplies pressurized fluid to the torque converter and transmission circuits.
  
• Transmission pressure regulator valve: A valve that maintains consistent hydraulic pressure within the transmission system.
  

Initial Symptoms and Field Diagnosis
In one restoration case, a 1975 Case 850 dozer was recovered after being submerged in a mud hole for an extended period. After draining and replacing all fluids multiple times, the machine could move forward and reverse but would stall intermittently. The driveshaft would stop spinning, and only by shifting to neutral and re-engaging gear could movement resume. Additional throttle helped slightly, but the machine lacked torque.
The operator inspected the torque converter visually and found the flex plate and converter still spinning during stall events. This suggested that the converter was not generating sufficient hydraulic pressure to transmit torque effectively.
Pressure Testing and Performance Metrics
To confirm the diagnosis, a pressure gauge was installed on the torque converter circuit. Initial readings showed:

• Idle pressure: 38 psi
  
• High RPM (1800–2000): 55 psi
  
• Warmed-up pressure: 25–40 psi
  

These values were significantly below expected operating ranges, which typically exceed 100 psi under load. Transmission pressure was also measured:

• Idle: 215 psi
  
• High RPM: 280–290 psi
  
• In gear: 255–260 psi
  

While transmission pressure remained relatively stable, the torque converter pressure dropped as the machine warmed up, indicating internal leakage or pump degradation.
Potential Causes and Component Analysis
Several factors may contribute to low torque converter pressure:

• Worn internal seals within the converter
  
• Charge pump wear or thermal expansion reducing efficiency
  
• Contamination from water intrusion damaging aluminum components
  
• Valve body clogging or erosion from sediment
  

In this case, the torque converter cavity was dry, as expected, and no external leaks were visible. However, the history of submersion raised concerns about internal corrosion and sediment infiltration.
A technician from Queensland noted that once mud or water enters the transmission system, complete flushing is nearly impossible. Contaminants embed in clutch packs, valves, and aluminum housings, causing persistent performance issues.
Component Replacement and Restoration Strategy
To restore full functionality, the following steps are recommended:

• Remove and disassemble the torque converter
  
• Inspect and replace internal seals, bearings, and vanes
  
• Replace the charge pump if wear is evident
  
• Flush all hydraulic lines and valve bodies with solvent
  
• Replace transmission filter and confirm check valve installation
  
• Install new gauges to monitor pressure and temperature
  

In one similar case involving a Case 1150 dozer, a full teardown and rebuild of the torque converter and transmission cost over $25,000. While expensive, it was the only solution after repeated failures and partial repairs.
Lessons from the Field and Operator Insights
The restoration operator shared that the dozer’s gauges had been smashed by a relative’s child, leaving no way to monitor pressure during operation. This underscores the importance of functional instrumentation in diagnosing hydraulic systems.
Another technician emphasized checking pressure across all gear ranges—low, high, forward, reverse—and comparing engagement times between tracks. Uneven pressure or delayed engagement can indicate clutch pack seal failure or shaft end cap leakage.
Recommendations for Future Use and Preservation
For owners of vintage Case dozers, especially those exposed to water or mud:

• Avoid operating submerged equipment without full inspection
  
• Replace fluids immediately after exposure and monitor for emulsification
  
• Install aftermarket pressure gauges for real-time diagnostics
  
• Document all repairs and pressure readings for future reference
  
• Consider full rebuilds if pressure remains unstable after flushing
  

Conclusion
The 1975 Case 850 dozer remains a durable and serviceable machine when properly maintained. However, water intrusion into the torque converter and transmission can cause persistent hydraulic failures that are difficult to resolve without disassembly. Through careful pressure testing, component inspection, and methodical restoration, even submerged machines can be brought back to life. For enthusiasts and professionals alike, the Case 850 offers a rewarding challenge and a glimpse into the robust engineering of mid-20th-century earthmoving equipment.